1. Field of the Invention
The present invention pertains to the field of coatings, more specifically to polymers crosslinked with blocked isocyanurates or blocked biurets of diisocyanates. The present invention pertains especially to automotive clearcoats including polymers crosslinked with blocked isocyanurates or blocked biurets of diisocyanates.
2. Background of the Invention and Material Information
In the field of automotive coatings, it has become an objective to obtain a clearcoat (i.e., the outermost automotive coating) that is resistant to being etched by environmental fallout. Environmental etching is manifested by pitting, water spotting or chemical spotting of the coating or any combination of these. Etch resistance is desirable because it improves the appearance and useful life of the coating.
Etch resistance can be measured by visual examination of the coating, or by a profilometer or by subjecting a coating on a test panel to a saline solution in a temperature gradient oven test. Etch resistance is generally measured by visual examination by individuals skilled at examining finishes for the degree of etch therein. Etch resistance has become more important as the amount of acid rain, and other industrial fallout has increased. Currently, environmental etching of automotive finishes is generally greatest in the areas most greatly affected by industrial fallout.
In addition to testing for resistance to environmental etching, automotive coatings are typically tested for resistance to damage from: exposure to ultraviolet radiation emitted from an ultraviolet emitting light bulb, exposure to ultraviolet radiation emitted from a xenon arc bulb, exposure to sunlight in regions differing in latitude, climate, and pollutant levels, exposure to high relative humidity at high temperature, and impacts made by small, hard objects.
Accordingly, it is most preferable to produce a coating having an optimum mix of characteristics with regard to all of the tests described above. In order to be commercially successful, a coating should test favorably in as many testing situations as possible. The sum of all of the characteristics of any particular coating determine its value in the real world of automotive coatings.
It is also desirable, in the manufacture and use of automotive coating formulations, to select compounds in order to minimize environmental impact and the expense involved in recovery thereof, and to utilize compounds that do not present an unacceptable risk to occupational safety or occupational health.
Clearcoat compositions used in the automotive industry are generally one component or two component compositions. A one component composition contains all ingredients of the composition in a single solution or dispersion. The one component composition forms films that can be cured via merely heating, without the use of any other solution or dispersion.
In contrast, a two component composition requires that two solutions or dispersions be mixed together to form the composition. Generally, once the solutions or dispersions are combined the composition is applied to the substrate as quickly as possible. Typically the components are mixed together immediately upstream of the nozzle of a sprayer and the mixture is immediately atomized by the nozzle into a mist that is directed at a substrate to form a film of the coating on the substrate.
Currently the most prevalent clear coat compositions utilized in the automotive industry are one-component clearcoat compositions. This is because most manufacturers employ paint booths designed for one component coating compositions. The most commonly used one coat compositions are melamine cure compositions. These compositions do not require that two or more reactive solutions or dispersions be mixed immediately before being applied to a substrate to be coated. These compositions crosslink upon exposure to heat in the presence of catalysts, and can be applied as one component compositions because they do not react until they are exposed to relatively high temperatures in the presence of a suitable catalyst.
However, one component compositions that comprise melamine are not without their disadvantages. It has been found that cured automotive clearcoats made using these compositions exhibit unsatisfactory environmental etch characteristics.
An alternative to the melamine cure, one component coating compositions, are one component compositions utilizing crosslinkers that include a blocked isocyanate functionality in combination with a film forming polymer. The term "blocked isocyanate" refers to the fact that the isocyanate reactive functionality --NCO-- is reacted with a blocking agent. Preferred blocking agents are the oximes. Other blocking agents include suitable aliphatic, cycloaliphatic, aromatic, alkyl and aromatic-alkyl monoalcohols. Additional blocking agents are phenols and substituted phenols where the substituents do not adversely affect the coating operations. Useful amine blocking agents include dibutyl amine and tertiary hydroxyl amines. The blocking agent prevents the --NCO-- functionality from reacting with a reactive functionality on the polymer until the composition is heated to a temperature at which the blocking agent volatilizes (deblocking temperature). When the blocking agent volatilizes, the reactive --NCO-- functionality on the crosslinker can then react with a reactive functionality on the polymer. The film forming polymer has at least one functional group reactive with an isocyanate functionality.
To form the coating, the crosslinker comprising the blocked isocyanate functionality is combined with the polymer and a liquid carrier in which the crosslinker and polymer dissolve and/or disperse. The resulting mixture must be stored at a temperature below the deblocking temperature so that the crosslinker cannot react with the polymer. The deblocking temperature typically ranges from 135.degree. C. to 155.degree. C. The one component mixture can be sprayed onto a substrate so that a film of the composition is formed on the substrate. The film of the mixture on the substrate is thereafter heated in an oven so that the blocking agent "deblocks" (i.e., is released), whereby the isocyanate is free to react with the polymer. The reaction of the isocyanate with the polymer results in the production of a crosslinked polymer network, i.e., a cured coating.
One component coating compositions comprising a blocked isocyanate typically use only a blocked isocyanurate of hexamethylene diisocyanate. The blocked isocyanurate of hexamethylene diisocyanate is known to be advantageous for this use because it produces a coating having a good hardness to flexibility ratio. The blocked isocyanurate of hexamethylene diisocyanate is one of a few isocyanurates that are soluble or dispersible in the solvents and dispersants used in making coatings. Furthermore, the blocked isocyanurate of hexamethylene diisocyanate is a relatively inexpensive component in comparison to other isocyanurates.
An alternative blocked isocyanurate for use in the one component coating compositions is a blocked isocyanurate of diisophorone diisocyanate. However, there are several problems with this crosslinking agent including decreased mar resistance and brittleness, due to the very hard coatings that result. Also, these coatings tend to degrade and crack upon exposure to ultraviolet radiation. There has also been a problem of yellowing in clear coat compositions utilizing these crosslinkers, making then suitable for clearcoat applications over only a dark basecoat.
The composition, process, and coating of the present invention are novel in that they are more etch resistant and mar resistant and they overcome problems of yellowing, brittleness and degradation usually encountered with coatings containing blocked isocyanurates of isophorone diisocyanates.